Add buildable C engine, shaders and music source

fx/fullscreen.hlsl

@@ -0,0 +1,18 @@
+// One triangle that covers the screen, built from the vertex id alone.
+// Nothing is bound: no vertex buffer, no input layout, no index buffer.
+// Draw(3, 0) with topology TRIANGLELIST is the whole call.
+
+struct VSOut
+{
+    float4 pos : SV_Position;
+    float2 uv  : TEXCOORD0;
+};
+
+VSOut main(uint id : SV_VertexID)
+{
+    VSOut o;
+    o.uv  = float2((id << 1) & 2, id & 2);       // (0,0) (2,0) (0,2)
+    o.pos = float4(o.uv.x * 2.0 - 1.0,
+                   1.0 - o.uv.y * 2.0, 0.0, 1.0);
+    return o;
+}